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Interpretive Summary: Wild carrot is a common weed throughout North America and much of the rest of the world, and it intercrosses readily with cultivated carrot. This is usually not a significant problem, since the typical carrot crop does not flower. But for carrot seed producers, wild carrot is a major problem since a commercial carrot seed crop which has been grown near wild carrot can easily get pollinated by insects that visit flowering wild carrots. The result is off-types that look like wild carrot, namely white forked roots. The outcrosses with wild carrot ruin the quality of the crop. Since these outcrosses look like wild carrot, they are easy to identify in a commercial carrot seed crop. However, it is not well know if pollen from the commercial carrots being grown for seed production is carried to nearby wild carrot – because these crosses look like wild carrot. The purpose of this study was to evaluate seed of various samples of wild carrot growing nearby and far from commercial carrot seed fields, and using molecular markers to fingerprint the DNA of plants in these seed samples, to ascertain if pollen from cultivated commercial carrot pollinates wild carrot. Based on the results, we can say with confidence that pollen from cultivated carrot does, in fact, get carried to nearby wild carrots, and the closer the wild carrot to the commercial seed field, the higher the incidence of pollen movement to wild carrot. This research is of interest to the vegetable seed industry, and to researchers studying the impact of agricultural practices on wild plants.

Technical Abstract: Studies of gene flow between crops and their wild relatives have implications for both management practices for farming and breeding as well as understanding the risk of transgene escape. These types of studies may also yield insight into population dynamics and the evolutionary consequences of gene flow for wild relatives of crop species. Moreover, the comparison of genetic markers with different modes of inheritance, or transmission, such as those of the nuclear and chloroplast genomes, can inform the relative risk of transgene escape via pollen versus seed. Here we investigate patterns of gene flow between crop and wild carrot, Daucus carota (Apiaceae) in two regions of the United States. We employed 15 nuclear simple sequence repeat (SSR) markers and one polymorphic chloroplast marker. Further, we utilized both conventional population genetic metrics along with species diversity indices of Shannon information theory as the latter have been proposed to be more sensitive to allele frequency changes and differentiation. We found that populations in both regions that were proximal to crop fields showed lower levels of differentiation to the crops than populations that were located farther away. We also found that Shannon measures were more sensitive to differences in both genetic diversity and differentiation in our study. Finally, we found indirect evidence of paternal transmission of chloroplast DNA and accompanying lower than expected levels of chloroplast genetic structure amongst populations. Our findings of substantial gene flow for both nuclear and chloroplast markers demonstrate the efficiency of both pollen and seed to transfer genetic information amongst populations of carrot.